Optical communication networks use a constellation diagram for the representation of a signal modulated by a digital modulation scheme such as quadrature amplitude modulation or phase-shift keying. The signal is represented as a two-dimensional diagram in the complex (I-Q) plane, where the possible symbols that may be selected by a given modulation scheme are represented as points (symbols) in the complex plane. In many cases, a constellation at the transmitting side must be converted to a corresponding constellation at the received side, in order to reconstruct the transmitted data. For example, if the transmitting side uses QPSK modulation scheme and the receiving side uses 8-PSK scheme, the QPSK constellation must be converted (mapped) to symbols in the 8-PSK scheme.
Such communication networks passed a significant breakthrough with the deployment of Erbium-Doped Fiber Amplifiers (EDFAs), which enabled all-optical amplification of transmitted signals. The all-optical amplification makes it possible to avoid electrical regenerators which employ Optical-Electrical-Optical (OEO) conversions for signal recovery. This is significant as OEO conversions require expensive high speed electronics, which becomes a limiting bottleneck in high transmission rates. In addition, OEO conversions require complex implementation as the optical signal should be detected, recovered, and re-transmitted. Since the development of the EDFAs, the field of all-optical signal processing has widely evolved, mainly due to the enabling technologies of photonic integrated circuits as well as highly nonlinear materials and devices. Consequently, all-optical devices have been developed to perform various operations such as optical switching, equalization, and application of logic functions. Another major advance in the evolution of optical communications is the development of coherent optical communication systems. These systems have high spectral efficiency and ability to engage digital signal processing at the receiver side, in order to compensate for the channel impairments. Since different coherent optical communication networks can use various constellations, a constellation conversion is often performed when an optical bit stream reaches a node connecting two optical communication networks, e.g., an optical transport network and a data center.
Currently, this conversion is performed by electrical regeneration which uses a bit-rate restrictive OEO conversion. Consequently, all-optical constellation conversion methods have recently been published introducing conversion from Non-Return to Zero (NRZ) to Return to Zero (RZ) format, NRZ to Binary Phase Shift Keying (BPSK), NRZ to Quadrature Phase Shift Keying (QPSK), On-Off Keying (OOK) to 16 quadrature amplitude-phase modulation (QAM), RZ-OOK to NRZ-QPSK and conversions among M-ary Phase Shift Keying (PSK) modulation formats. However, all prior art solutions failed to provide a generic method for constellation conversion.
It is therefore an object of the present invention to provide a method and system for symbol constellation conversion, which are generic and can convert any given constellation of symbols to any desired constellation.
It is another object of the present invention to provide a method and system for symbol constellation conversion, which are all optical and do not require expensive electronic fast sampling.
It is an additional object of the present invention to provide a system for symbol constellation conversion, which may be implemented on a compact silicon integrated circuit.
Other objects and advantages of the invention will become apparent as the description proceeds.